Electromagnetic tracking (EMT) technology for improved treatment quality assurance in interstitial brachytherapy

Abstract Electromagnetic Tracking (EMT) is a novel technique for error detection and quality assurance (QA) in interstitial high dose rate brachytherapy (HDR‐iBT). The purpose of this study is to provide a concept for data acquisition developed as part of a clinical evaluation study on the use of EMT during interstitial treatment of breast cancer patients. The stability, accuracy, and precision of EMT‐determined dwell positions were quantified. Dwell position reconstruction based on EMT was investigated on CT table, HDR table and PDR bed to examine the influence on precision and accuracy in a typical clinical workflow. All investigations were performed using a precise PMMA phantom. The track of catheters inserted in that phantom was measured by manually inserting a 5 degree of freedom (DoF) sensor while recording the position of three 6DoF fiducial sensors on the phantom surface to correct motion influences. From the corrected data, dwell positions were reconstructed along the catheter's track. The accuracy of the EMT‐determined dwell positions was quantified by the residual distances to reference dwell positions after using a rigid registration. Precision and accuracy were investigated for different phantom‐table and sensor‐field generator (FG) distances. The measured precision of the EMT‐determined dwell positions was ≤ 0.28 mm (95th percentile). Stability tests showed a drift of 0.03 mm in the first 20 min of use. Sudden shaking of the FG or (large) metallic objects close to the FG degrade the precision. The accuracy with respect to the reference dwell positions was on all clinical tables < 1 mm at 200 mm FG distance and 120 mm phantom‐table distance. Phantom measurements showed that EMT‐determined localization of dwell positions in HDR‐iBT is stable, precise, and sufficiently accurate for clinical assessment. The presented method may be viable for clinical applications in HDR‐iBT, like implant definition, error detection or quantification of uncertainties. Further clinical investigations are needed.


| INTRODUCTION
Interstitial brachytherapy (iBT) is an established treatment option for treating cancer in numerous body sites including, e.g., prostate, 1 head & neck, 2 breast, 3 and gynecology. 4 For breast treatments accelerated partial breast irradiation (APBI) is proven to have at least comparable outcome to established whole breast treatment schemes. [5][6][7] In current clinical practice, catheter positions are reconstructed based on computed tomography (CT), magnetic resonance tomography (MRT) or ultrasound imaging data. 8 The accuracy of the reconstructed catheters depends on the imaging modality, but can be hampered by imaging artifacts, the limited slice thickness of CT/ MRT data which typically ranges from 2 to 5 mm for brachytherapy planning, the individual anatomy of the patient, the treatment site, or approximations such as deviations between the reconstructed and the actual source path that can lead to discrepancies up to 5.5 mm. 9 In addition, the current clinical procedures for implant reconstruction are time-consuming and observer-dependent. 10 In parallel to the clinical success of brachytherapy, efforts are increasing to address and reduce uncertainties and to detect and avoid errors in brachytherapy treatments. 9,11,12 Like in other high precision therapy options such as stereotactic (external beam) radiation therapy, for which, e.g., intensity modulated treatment plans are verified in dosimetry phantom before treatment, 13 these errors could very likely be identified prior treatment. Among the most common medical events reported by the Nuclear Regulatory Commission (NRC) in the United States related to high-dose-rate (HDR) treatment planning are: wrong indexer length, catheter reconstruction errors, and misidentified first dwell position. 11 One option for verifying the 3D implant geometry and the corresponding dwell positions is electromagnetic tracking (EMT) as described in the following.
EMT with miniaturized electromagnetic sensors is widely used in clinical practice. Examples are tracking of instruments in surgical interventions, 14 guidance of biopsies, 15 and motion monitoring in ablation 16 or external beam radiation therapy. 17 A review of the technological fundamentals and the clinical applications is given by Franz et al. 18 In the last years, EMT was also proposed for interstitial brachytherapy mainly addressing implant reconstruction and error detection, 19 i.e., applications in-line with the ongoing efforts to increase quality assurance (QA) in brachytherapy.
Several studies in phantoms exploring the use of EMT in HDR-iBT have focused on implant reconstruction. 10,20,21 Zhou et al. addressed HDR treatments of prostate cancer with the goal to increase the accuracy and speed of the implant tracking compared to ultrasound images based reconstruction as the current clinical standard. 20 Using a calibration phantom, they assessed the noise level and tracking accuracy in their operating room, including the known interferences of EMT and (ferromagnetic) metals in the clinical environment. They report an accuracy of 1.6 AE 0.2 mm in the operating room, supporting the findings of Nixon et al. 22 that especially the distance of the field generator to the sensor should be minimized. The authors concluded that EMT-based implant reconstruction is faster and more accurate than the ultrasound-based procedure. Similar conclusions have been drawn by Bharat et al.,10 who reported agreement in EMT, CT, and TRUS-identified implant geometries, and Poulin et al., 21 who found that EMT based reconstruction of the implant is more accurate than CT.
The focus is shifted toward EMT-based error detection for HDR-iBT in the work of Damato et al. 23 In a phantom study, they mimicked swapping, wrong intersecting, and shifting of catheters and tried to identify the introduced errors by EMT. Identification of catheter swapping and wrong intersections was successful in all studied cases. Shifts could be identified with 100% sensitivity and specificity if their magnitude was > 2.6 mm.
We aim to assess the clinical feasibility of error detection and monitoring of the implant geometry by EMT in interstitial HDR breast cancer treatments. For interstitial multicatheter brachytherapy after breast-conserving surgery, the standard HDR treatments at the University Clinic Erlangen, Germany are currently planned using CTbased implant reconstruction and delivered in nine fractions within 5 days.
The purpose of this study was to provide a technical description of the data acquisition and processing procedure that was developed as part of a clinical evaluation study on the use of EMT during interstitial treatment of breast cancer patients. The procedure includes a protocol for compensation of breathing motion that influences the catheter EMT measurements. Based on the data acquisition technique the stability, accuracy, and precision of EMT-determined dwell positions were quantified.

2.A | The EMT system
In this study, the third generation Aurora â Electromagnetic Tracking System from Northern Digital Inc. (NDI, Waterloo, Canada) was used. The system's field generator (FG) was mounted on a flexible and lockable position arm over the phantom placed on a patient bed, scanner table or treatment table (Fig. 1c). The FG covers a cubic tracking field of 500 9 500 9 500 mm 3 . The positions of four sensors were tracked at the maximum measuring rate of 40 Hz. In this study we had interest in the positions, p = (x, y, z) in R 3 , of one 5 degree of freedom (DoF) sensor (Aurora 5DoF and to simultaneously carry out the initial data analysis routines.

2.B | The quality cross-check phantom
To assess the precision and accuracy of the EMT system in its clinical brachytherapy mode of operation, we constructed a phantom that will also be used for regular QA purposes (see Fig. 1). The design is inspired by the so-called Baltas phantom. 24 In this study, we only used the geometric catheter configuration of our phantom.
The 120 9 120 9 60 mm 3 phantom is built out of six polymethyl methacrylate (PMMA) plates (each with a height of 20 mm).
A total of eight plastic catheters (6F Flexible Implant Tubes, Elekta Brachytherapy Veenendaal, The Netherlands) are linearly guided by grooves of the aligned and fixed PMMA plates (see Fig. 1). Four of the catheters are centrally arranged between the middle plates at a (center-to-center) distance of 20 mm from each other to obtain the usual pitch of the holes in the template used for breast implants.
Parallel to this level at a right angle (90°and À90°), each 20 mm above and below, two catheters are placed at a distance of 40 mm to each other. The manufactured phantom dimensions were measured by a calibrated caliper (AE 0.01 mm) and formed the basis for the reference dwell positions (Section 2.D).
The phantom was also used to determine the measuring location of the implant sensor relative to its physical tip [used as distance the phantom can be compared with the distance d EMT. termined by EMT, yielding d sensor via: The material PMMA has been chosen to establish a setup for EMT that resembles the measurements of breast patients with implanted catheters. Based on Biot-Savart's law 25 concerning the magnetic flux, for which the used sensor type is sensible, the expected position shift Dp in the distance r 0 from the magnet source follows: with l r being the relative magnetic permeability of the penetrated medium. Considering water (l water r = 0.999 991) as a medium for the human body, the calculated position shift due to a magnetic field interference is Dp r0 ¼ 3 Á 10 À6 . The relative magnetic permeability of PMMA (l PMMA r . 1 deviates insignificantly from water. For both media, the expected position shift Dp therefore is well below (≲ 1&) the accuracy in air of ≤ 2 mm according to the accompanying calibration protocol (Planar 20-20 Field Generator, 2015-01-26, NDI Europe GmbH, Radolfzell, Germany) of the used EMT system. PMMA thus mimics the human body, so for both phantom and patient measurements correction to account for the measuring medium is omitted.  the patient and staff if completed within 10 min. Each measurement started with the implant sensor fully inserted to the tip-end of the catheter. After initiating the EM data acquisition, the sensor is held at this position for~2 s until an acoustic signal indicates to the operator that retracting may start. This procedure assures a reliable determination of the tip-end (see also below).

2.C | Raw data analysis
The measured EMT-raw data (see Fig. 2 according to: Following our clinical routine for breast treatment planning, an offset s offset of 5 mm is introduced from the button center to the first dwell position P k i (k = 1) in R 3 . The next dwell positions P k i ðk ¼ f2; . . .; 48gÞ are determined by the step size of s step = 2.5 mm. Considering the distance d sensor from the physical sensor cable tip to its individual measuring location (determined by QA routines using the introduced phantom, see Section 2.B) and the distance d i;catheter from the tip-end of each catheter i to its button center (determined by measuring with a precision caliper), the distance d i from P start i to the first dwell position P 1 i is determined according to: Within the distance range of d i AE s step the mean position of the located p j i ðt i Þ was taken to identify the direction vector u 1 i for the linear equation determining the first dwell position P 1 i (see Fig. 2(a)): The button center P 0 i was determined into the opposite direction with our clinically usual offset to P 1 i : The next dwell positions P k i were determined accordingly: whereby P kÀ1 i is the current position and u k i is the direction vector, determined by the mean position over the located P j i in the distance of 2 Á s step .   For each measurement used in the study, the setup is displayed in Fig. 1

2.E.1 | Precision
The term precision is used (in accordance with ISO/IEC Guide 99:2007) 31  According to the Shapiro-Wilk test~10 % of the measurements were not normally distributed (H = 0, P = 0.05, n~1/10). The measured data were thus mainly analyzed based on the 50th (median), 95th and 99th percentile. As suggested by Nixon et al., 22 the median and the 95th percentile were fitted by adjusting k to

2.E.3 | Accuracy
The term accuracy was used (in accordance with ISO/IEC Guide 99:2007) 31 as "the closeness of agreement between a measured quantity and a true quantity value of a measurement". The determined dwell positions were used as "measured quantity", the reference dwell positions based on the fixed catheters guided in the phantom with known geometry were interpreted as "true quantity value". This definition of the accuracy encompasses the interpretation of the measured positions including the registration as described in Section 2.C.

| Precision
To assess the precision of the used EMT at the different settings 50th (median), 95th and 99th percentile were analyzed on the three different clinical tables. Based on grouping the following results were found: On the CT table, the 95th percentile reached up to 0.12 mm (0.34 mm for 99th percentile; 0.03 mm for median) over all distances FG to catheter (d FG ) and heights of Styrodur â pads (d table ).
For measurements at d FG ffi 100 mm, the EMT system failed due to a high tracking error signaled by the EMT system most likely due to interferences with the metallic components of the   The data for median and 95th percentile are fitted following the approach of Nixon et al. 22 Details for the different settings can be found in Table S1 (available in the Supplementary Materials file available on the JACMP website at www.jacmp.org).  Table 1) only the following ones showed an effect: Metallic components close to the tracking field (label E and G, noise level of 0.17 mm and 0.57 mm, respectively) and shaking of the FG (label L, noise level of 1.1 mm). Since the different spatial directions did not show an influence on the precision, the data were gathered for each setting of the movement velocity.

| Accuracy
Accuracy was studied in the same settings as precision. The median over each catheter measurement at the CT The linear polynomial (fit8) and the approach of Nixon et al. 22 (fit9) are fitted to the median data (see Fig. 6). Based on these fits accuracy data for different settings are provided in

| Error detection
Swapping of catheters was detected (case a). The deviation at the 1st dwell positions are close to the measured geometrical distance (= 20.0 mm), see Table 2. The gradual displacement of a single catheter (case b) showed that the geometrical shifts Δl are close to the determined distances, see Table 3. Over all, both examples for error detection were below the determined accuracy (95th percentile of 0.83 mm).

| DISCUSSION
A number of studies showed that EMT is a viable approach for quality assurance and implant tracking in brachytherapy. 19 This study is the technical foundation of a recently initiated clinical investigation to study potential changes of the implant geometry in interstitial HDR brachytherapy of breast cancer by EMT. To allow drawing conclusions from this clinical investigation, a sound data acquisition and reconstruction workflow had to be established aiming at a precision and accuracy level sufficient for that purpose.     Table 1. The measured distance to the coordinate origin defined by the fiducials is plotted on the left axis, whereby the minimum is shifted to zero millimeter. The maximum fluctuation per minute section is plotted as noise level on the right axis. F I G . 5. Dynamic precision. Linear fit functions over the median, 95th and 99th percentile of the movement velocities from 0 mm/s to 60 mm/s. The constant coefficient was fixed to the determined value at rest (0 mm/s).
distances (d FG \ 120 mm). The precision values of all studied d FG to d table combinations on all three clinical tables are < 0.34 mm for the 99th percentile, that is, lower than the source precision of AE 1 mm specified by the manufacturers. 9 Only metallic objects such as a junction box with metallic components lowered from the ceiling or sudden shaking of the FG influenced the precision.
Since EMT will be used for breast implant measurements, the reading of the implant sensor needs to be corrected with respect to overlaid breathing motion. Currently, the mean of the three fiducial sensor's position is used for that purpose, but more sophisticated correction protocols are likely needed since the mean of the fiducial sensors will most likely not precisely reflect the motion influence at the position of the implant sensor. In section 3.B, the precision under the influence of motion was determined. We observed a linear dependency on the motion velocity. But even at 60 mm/s, which is more than the expected velocities due to breathing motion, 36 the 95th percentile reaches only 0.19 mm which is tolerable for the clinical goals. In previous studies, for example, of Nafis et al. 37 higher values were reported (0.54 mm at 50 mm/s) potentially due to more irregular motion patterns.
With respect to accuracy, we chose a precision built phantom as ground truth to be independent of the limited resolution of imaging data and/or the limited calibration of the imaging device. The effect of such influence was reported by Poulin et al. 21 They determined that the identification error of the sensor tip changed from 0.69 mm to 1.08 mm when the ground truth changed from lCT to CT, respectively. The accuracy was more influenced by the distance from the bed/ table (d table ) than the precision (see Fig. 6). Especially on the CT     to date indicate that the deployed motion compensation is useful when applied to patient data. Clinical implementation on a daily level needs further investigations with respect to reference data in case error or uncertainty detection is the aim of the implementation. A smooth implementation of such a system into the clinical workflow, that is, proper placement of the field generator, automatic recording of EMT data, for example, by incorporating the sensor into an afterloader are desired. Furthermore, appropriate quality assurance methods of the EMT system including site-checks to spot metallic components potentially influencing the EMT acquisition need to be developed.

This study was supported by an unrestricted research grant from
Elekta.

CONF LICT OF I NTEREST
Although the co-author V. Strnad is a consultant to Elekta, Veenendaal, The Netherlands, there is no conflict of interest.

SUPPORTING INFORMATION
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